The present invention relates to an apparatus and method for reducing noise in a video system by applying motion compensated temporal filtering using previously generated motion vectors and adaptive spatial filtering at scene change frames.
Digital television offers viewers high quality video entertainment with features such as pay-per-view, electronic program guides, video-on-demand, weather and stock information, as well as Internet access. The video images, packaged in an information stream, are transmitted to the user via a broadband communication network over a satellite, cable, or terrestrial transmission medium. Due to bandwidth and power limitations, efficient transmission of film and video demands that compression and formatting techniques be extensively used. Protocols such as MPEG1 and MPEG2 maximize bandwidth utilization for film and video information transmission by adding a temporal component to a spatial compression algorithm.
Each individual image in a sequence of images on film or video is referred to as a frame. Each frame is made up of a large number of picture elements (pixels) that define the image. Within each frame, redundant pixels describe like parts of a scene, e.g. a blue sky. Various types of compression algorithms have been used to remove redundant spatial elements thereby decreasing the bandwidth requirements for image transmission. Sequences of frames on film or video often contain pixels that are very similar or identical as well. In order to maximize bandwidth utilization, compression and motion compensation protocols, such as MPEG, are typically used to minimize these redundant pixels between adjacent frames. Frames referenced by an encoder for the purpose of predicting motion of images within adjacent frames are called anchor frames. These anchor frames can be of type Intra-frame (I-frame) or Predicted-frame (P-frame). Groups of pixels (macroblocks) that are mapped without reference to other frames make up I-frames, while P-frames contain references to previously encoded frames within a sequence of frames. A third type of frame referred to as a Bi-directional (B-frame) contains macroblocks referred from previously encountered anchor frames and macroblocks from anchor frames that follow the frame being currently analyzed. Both B-frame and P-frame encoding reduce duplication of pixels by calculating motion vectors associated with macroblocks in a reference frame, resulting in reduced bandwidth requirements. The choice of encoding type for a particular frame is dependent upon the complexity of that image.
For images that pan, pixels that describe moving objects are largely the same, in that they are only spatially displaced. Instead of repeatedly specifying these pixels in consecutive frames, it is often advantageous to reference groups of them, i.e. macroblocks, in previous (or forthcoming) frames. A motion vector directs the video processor where to obtain the macroblock in a referenced frame. The use of motion vectors for this purpose is referred to as motion compensation. Motion compensation can also be exploited to help reduce the effect of noise in encoded video images.
Various types of noise can be introduced into video prior to compression and transmission. Artifacts from the imaging and recording equipment, from terrestrial or orbital transmission equipment, from communication channels, and from encoding and decoding equipment are well known. Noise introduced prior to image compression is problematic because it interferes with the performance of subsequent compression systems by monopolizing data bandwidth while decreasing video quality. Additionally, quantizing in the Discrete Cosine Transform (DCT) domain tends to magnify the effects of noise leading to increased signal degradation.
While filtering reduces noise in a video image, it can, consequently, reduce the resolution (e.g. sharpness) of the image, leading to imprecise edge transitions, thereby reducing apparent focus. An edge is defined as an abrupt change in pixel amplitude such as a color difference and/or luminance amplitude change between sets of pixels. These abrupt changes are typically oriented in a vertical or horizontal direction, such as an edge between a blue sky and a black building.
Accordingly, there is a need for an improved noise filtering system that would reduce many of the disadvantageous effects found with contemporary digital image filters. The present invention provides a solution for solving these problems while simultaneously providing enhanced throughput of film or video frame encoding.